This is the rulings of Ayatullah Sistani (May Allah protect him)
Ruling 378. In ghusl, short hair that is considered part of the body must be washed. It is not obligatory to wash long hair. In fact, if one makes water reach the skin in a way that the hair does not become wet, the ghusl is valid. However, if it is not possible for water to reach the skin without the hair becoming wet, then one must wash the hair in a way that water reaches the body.